This invention relates to laser tracking optics, and more particularly, to a beam chopper for use within a time shared aperture device.
There are several means of sampling radiant energy beams. These include spectral-sharing devices using gratings, beam splitters in an optical system or refractive elements, or spatial-sharing devices using different areas of the aperture for beam projection and for return viewing of a portion of the projected beam modified by the presence of a target, etc.
As to the spectral-sharing device, a high energy laser beam reflected by a target is not used for tracking because of interference from scattering of the outgoing beam in the beam expander optics. Instead, an offset wavelength near the laser beam wavelength is used for tracking, etc. The return having this offset wavelength is created either by an independent laser illuminator or the spectrum spreading of the laser beam off the target. This offset wavelength returns through the same aperture as the outgoing laser beam thus resulting in spectral-sharing of the same aperture.
In particular, a prior art spectral-sharing device has a high energy laser that emits a beam which is incident on a first reflective grating. The laser beam is further incident upon a second reflective grating. A portion of the beam incident upon the second reflective grating is reflected into a waveform analyzer, and the remaining portion is transmitted through beam expander optics. If a target is within the transmitted beam, a portion is returned through the beam expander optics. Because of the above problem with interference, an offset wavelength different than the transmitted wavelength is analyzed. The waveform analyzer of the returned beam receives the returned beam off the second grating. Because of the offset wavelength, the target source can be weak and undependable especially against a background of other sources of radiation such as stars, atmospheric condition, etc.
A spatial-sharing device has a high energy laser that emits a laser beam into an optical train. From the optical train, the beam goes into a beam expander. A target return, having the same wavelength as the laser beam, traverses the same beam expander and optical train. A means is furnished so that the target return is analyzed but this analyzing means remains in the optical path of the laser beam all of the time so that there is a reduction in the size of the aperture useable by the laser beam.
Each of the above devices have disadvantages. One receives a target return at an offset wavelength. The target source reflects a return which can be weak and undependable especially against an unpredictable background. This limits the bandwidth of the target return and the precision obtainable for sensing figure errors and tracking. The other device receives a target return at the laser wavelength. The analyzing means for the target return blocks part of the aperture that could be used by the outgoing laser beam. This reduces the intensity of the laser beam that reaches the target.